1. Field of the Invention
The present invention relates to a Chemical Vapor Deposition (hereinafter referred to as CVD) method. More particularly, the present invention relates to a CVD method suited to mass production of oxide films having favorable characteristics, especially suited to mass production of oxide films for gate and having favorable characteristics.
2. Description of the Related Art
As a manufacturing method of liquid crystal display, a method of using high temperature polysilicon TFT (thin film transistor) and a method of using low temperature polysilicon TFT have been known. In the manufacturing method of using high temperature polysilicon TFT, in order to obtain a silicon oxide film of high quality, a quartz substrate which can be fit for a high temperature exceeding 1000xc2x0 C. is used. By contrast, in manufacture of low temperature polysilicon TFT, an ordinary glass substrate for TFT is used, so that it is necessary to form a film at low temperature (for example, 400xc2x0 C.). The manufacturing method of liquid crystal display by using low temperature polysilicon TFT has an advantage that its manufacturing cost is small, since it dose not require any special substrate. So that, it is hence widely employed recently, and its production is expanding.
In manufacturing of liquid crystal display by using low temperature polysilicon TFT, and forming a silicon oxide film appropriate as gate insulating film at low temperature, plasma enhanced CVD is used.
When forming a silicon oxide film by the plasma enhanced CVD, silane or tetraethoxy silane (TEOS), etc. are used as representative material gas. The material gas is generally used in a state of adding carrier gas such as helium (He), etc., and hereinafter it is merely referred to as the material gas.
When forming a silicon oxide film by plasma enhanced CVD, using silane or the like as material gas, in a conventional plasma enhanced CVD system, the material gas and oxygen are introduced in the front space of the substrate, and then, plasma is produced by mixed gas of material gas and oxygen, and the substrate is exposed to the plasma, thereby a silicon oxide film is formed on the surface of the substrate. In such a conventional plasma enhanced CVD system, the material gas is directly supplied into the plasma produced in the plasma enhanced CVD system. Accordingly, in the conventional plasma enhanced CVD system, ions of high energy are injected from the plasma exiting in the front space of the substrate to the film forming surface of the substrate, and the silicon oxide film is damaged, so that, film propertied are impaired. Further, since the material gas is directly introduced into the plasma, the material gas and plasma react violently with each other to generate particles, thereby lowering the yield.
To solve the problems, in the previous Japanese Patent Application (unexamined Japanese Patent Publication No. JP P2000-345349A), it has been attempted to improve the conventional plasma enhanced CVD system, and a new CVAD system was proposed.
The CVD system proposed in JP P2000-345349A is a system for producing plasma in vacuum container to generate neutral exciting radicals, that is to say to generate exciting radicals, and forming a film on the substrate by the said exciting radicals and materials gas. A conductive partition wall is disposed in the inside of the vacuum container of the said CVD system. Thereby, the inside of the vacuum container is separated by the conductive partition wall into two compartments. One of these two compartments is formed as a plasma generating space containing high frequency electrode, and the other is formed as a film forming space with a substrate holding mechanism for mounting substrate. The conductive partition wall has plural penetration holes for communicating between the plasma generating space and film forming space, and also has an inner space separated from the plasma generating space and communicating with the film forming space through plural diffusion holes.
In a CVD method conducted by a CVD system proposed in JP P2000-345349A, the material gas is supplied from outside into the inner space of the conductive partition wall, and is introduced into the film forming space through the plural diffusion holes. On the other hand, exciting radicals formed in the plasma generating space are introduced into the film forming space only through the plural penetration holes opened in the conductive partition wall. And, in the film forming space, a film is formed on the substrate by the exciting radicals and materials gas introduced into the film forming space as the before described.
In the CVD system proposed in this JP P2000-345349A, concerning the size (length and diameter) and structure of the penetration holes and diffusion holes, the penetration holes are defined in the size length and diameter) and structure so that the material gas introduced in the film forming space may not diffuse reversely into the plasma generating space, and the diffusion holes are defined in the size (length and diameter) and structure so that the exciting radicals introduced in the film forming space may not diffuse reversely into the inner space of the conductive partition wall.
That is, in the CVD system proposed in JP P2000-345349A, the condition of uL/D greater than 1 is satisfied, where u is the gas flow velocity in penetration holes, L is the substantial length of penetration holes (see FIG. 3, in this case, L is the length of the portion of the minimum diameter), and D is the binary diffusivity (mutual gas diffusion coefficient of two types of gases of material gas and process gas, for example, silane gas and oxygen gas). Concerning with the diffusion holes, too, the same condition as in the penetration holes is applied.
By the CVD system proposed in JP P2000-345349A, worsening of film properties of silicon oxide film formed on the glass substrate can be prevented, and the product yield can be improved.
Generally, silicon oxide film formed on a substrate by using exciting radicals, which are produced by generating plasma in a vacuum container, and material gas contains OH, hydrogen atom, or excessive silicon in the thin film (silicon oxide film) or in the lower interface of the thin film (silicon oxide film). The said OH, hydrogen atom, etc. may deteriorate the characteristics of silicon oxide film, which are required as insulating film. For example, the said deterioration of characteristics may include any increase of leak current, and hysteresis in capacitance-voltage curve.
It is hence an object of the present invention in manufacture of large-sized liquid crystal display using low temperature polysilicon TFT, to provide a CVD method capable of improving the film properties by using the CVD system newly proposed in JP P2000-345349A, which has successively prevented reverse diffusion of material gas into the plasma forming region, in the case of forming silicon oxide film on a substrate of a large area by using material gas such as silane, on the basis of the CVD making use of plasma.
To achieve the object, the present invention provides the following CVD method.
That is, the CVD method according to an aspect of the present invention is a CVD method for generating plasma in a vacuum container to produce exciting radicals, and forming a film on a substrate by the said exciting radicals and material gas. The CVD system to which this method is applied is composed as follows. That is, the inside of the vacuum container of the CVD system is separated into two compartments by a conductive partition wall, and one of the two separated compartments is formed as a plasma generating space containing a high frequency electrode, and the other compartment is formed as a film forming space containing a substrate holding mechanism for mounting substrates. The conductive partition wall has plural penetration holes for communicating between the plasma generating space and film forming space. And the conductive partition wall also has an inner space separated from the plasma generating space and communicating with the film forming space through plural diffusion holes.
In the CVD method conducted by the CVD system newly proposed in JP P2000-345349A as the before described, the material gas supplied from outside into the inner space of the conductive partition wall is introduced into the film forming space through the plural diffusion holes, and a high frequency electric power is applied to the high frequency electrode to generate a plasma discharge thereby generating exciting radicals in the plasma generating space, and the exciting radicals generated in the plasma generating space are introduced into the film forming space through the plural penetration holes in the conductive partition wall, and a film is formed on the substrate by the introduced exciting radicals and material gas in the film forming space.
The CVD method of the present invention conducted by the CVD system newly proposed in JP P2000-345349A as the before described, is characterized that it comprises a first step of forming a film on the substrate by the exciting radicals and material gas introduced in the film forming space, and a second step of cutting off the material gas supplied from outside into the inner space of the conductive partition wall to zero flow rate, and emitting the exciting radicals introduced in the film forming space through the plural penetration holes of the conductive partition wall to the thin film formed at the first step.
The CVD system having the above configuration to which the CVD method proposed by the present invention is applied is newly proposed in JP P2000-345349A. In this CVD system, plasma is generated by using oxygen gas, and a thin film is deposited on the surface of a substrate by using material gas such as silane and exciting radicals generated by the plasma. And, the inner space of the vacuum container, which is used as the treating compartment, is separated by a conductive partition wall into a plasma generating space for generating plasma and a film forming space. So that the treating surface of the substrate disposed in the film forming space is not exposed to the plasma. Being separated by the conductive partition wall, moreover, the material gas introduced in the film forming space is sufficiently suppressed from moving to the plasma generating space side. That is, plural penetration holes are formed in the conductive partition wall, and the plasma generating space and the film forming space existing at both sides of the conductive partition wall communicate with each other only through the said penetration holes, but the penetration holes are defined in the size and structure for preventing the material gas introduced in the film forming space from diffusing reversely to the plasma generating space side.
The characteristic structure of the CVID system proposed in JP P2000-345349A as the before described can be used for cutting off introduction of material gas into the inner space of the conductive partition wall, which separating the inner space of the vacuum container into the plasma generating space and film forming space, and selectively emitting only the electrically neutral exciting radicals, produced by the oxygen plasma generated in the plasma generating space, to the glass substrate disposed in the film forming space. And the said characteristic structure is suited to realizing it easily.
Accordingly, the present invention is intending to sufficiently use the before described characteristic structure of the CVD system proposed in JP P2000-345349A as well as to remove OH, hydrogen atom, and excessive silicon existing in the thin film or in the lower interface of the thin film. If OH, hydrogen atom, excessive silicon, etc. are existing in the thin film or in the lower interface of the thin film, it is considered that the characteristics of silicon oxide film is deteriorated, although the least amount of contained OH, hydrogen atom, and excessive silicon is required for silicon oxide film when it is used as insulating film.
According to the present invention, at the second step, by emitting exciting radicals successively to the film, which has been formed on the substrate by the exciting radicals and material gas introduced in the film forming space during a first step, the oxidation reaction can be promoted sufficiently while avoiding the risk of impact of the high energy ions to the silicon oxide thin film from the plasma.
Thus, the above problems are solved by the CVID method of the present invention for completing the process comprising the before described first step and second step.
As clear from the explanation herein, in the CVD method for forming a silicon oxide film or the like by using material gas such as silane, etc. by the plasma CVD on a substrate of a large area, for example, in the CVD method using a CVD system in which the inside of a vacuum container of the said CVD system is separated into a plasma generating space and a film forming space by a conductive partition wall having plural penetration holes, exciting radicals produced in the plasma generating space are introduced into the film forming space only through the said penetration holes, supplying material gas from outside into an inner space of the said conductive partition wall, which is separated from the plasma generating space and communicating with the film forming space through plural diffusion holes, and introducing the said material gas into the film forming space through the said diffusion holes, thereby thin film is formed on the substrate by the exciting radicals and material gas thus introduced into the film forming space, according to the present invention, thin film having excellent characteristics may be mass produced, by adopting the process of the present invention comprising a first step of forming a film on the substrate as the before described, and a second step of emitting exciting radicals to the film formed at the first step to promote oxidation reaction.
For example, in the oxide film for gate manufactured by employing the CVD method of the present invention comprising the before described first step and second step, the maximum hysteresis (maximum value of voltage difference at voltage-up characteristics and voltage-down characteristics) in the capacitance-voltage curve is improved from 1.2 V to 0.3 V as compared with the process of omitting the second step of the invention, that is, omitting the step of emitting exciting radicals.
The CVD system to which the CVD method of the present invention is applied can convey the electrically neutral exciting radicals produced in the plasma generating space selectively into the film forming space as mentioned above. Therefore, it is a great merit that the silicon oxide film in the process of forming at the first step is not exposed to the plasma containing high energy particles. Moreover, structurally, the material gas can be introduced only into the space opposite to the glass substrate, that is only into the partition wall, so that it is easy to change over lead-in and cut-off of material gas, and it is easy to control the gas content in the vacuum container, and hence the emitting process of exciting radicals at the second step can be also carried out effectively. That is, the CVD method of the present invention can make the best of the features of the CVD system to which this method is applied.